Then I removed the soldered copper tape again (which probably overstressed the 836 filter, see below), and solder a tiny copper plate on top (plate pressed flat on the surface filter, only soldered the sides while pressing down). This improved the result on the 1842 filter to only -3dB (was -10dB before), my best result so far. Unfortunately no more output on the 836...
Soldering surfaces is a risky idea. The filter is not soldered from the bottom to the PCB. PCB for RF doesn't like high temperatures. After all, the result of -3.5dB is quite a good result. It is not known how linear the generator is, in my CMU200 it has a deviation of about +/- 0.5 dB for -30dBm in the entire frequency range.
Yesterday I checked the first channel of my CRTU. The worst loss on an internal loop 1->1 is -1.5dB and 2->2 is -2dB. External loop 1->4 is -2.1dB and 3->2 is -2.4dB. The loss of -2.1dB is repeated for -35dBm in loops 1->4 and 3->2, which excludes problems with RF inputs. Probably the 836 MHz filter has too much loss, there is some fault or the module requires calibration. I don't have new 836MHz filters to verify this.
I add photos of filters before and after cleaning from the first channel of my CRTU. I cleaned the upper surface of the filters with a cotton swab and isopropylene alcohol, then a mixture of methyl alcohol, water and detergent (dishwashing concentrate), neutralized with isopropylene alcohol, then cleaned with ketchup (it cleans copper perfectly) and neutralized again with isopropylene alcohol. I isolated the filters from the RF absorber with laminating pouches (0.15mm). I did not want to stick anything to the surface of the filters so as not to damage the copper layer.
I will check the results of the loop measurements after the initial repair in the CMU after the return from the buyer (he replaced the rx / tx board and more
![Rant :rant:](https://www.eevblog.com/forum/Smileys/default/rant.gif)
).